Mobiles are often driven by external wind currents, but also by external or internal motors.
Mobiles often have the property of recursive or fractal self-similarity: a sub assembly may have a similar structure as the larger assembly it is a part of.
Each element acts as a beam suspended at a balance point in the middle, with one or more recursive elements suspended from the beam. The balance point of an element balances the gravitational mass of all the elements recursively suspended below it. Most mobiles have further recursive elements suspended from one or more ends of the beam. A simple counterweight at one end of a beam may terminate recursion. Some mobiles have further recursive elements suspended from the balance point of a beam, so that the balance points are one above the other in a vertical line.
Mobiles often have elements that rotate or pivot in horizontal planes but elements also may rock or seesaw in vertical planes.
A mobile may have flat, 2D elements that may be rotated so that the entire assembly lies flat in a vertical plane, but mobiles may also have 3D elements that are balanced plates instead of beams.
Mobiles and stabiles may be connected so that relative motion between elements is limited to one or more axis and is limited in range. For example mobiles may be connected by swivels that do not limit rotation or by hangers that do limit rotation. The connection between elements may be a bearing, a swivel, a knife-edge, a hook and eye, or other pivot.
In most mobiles, elements do not normally collide but may collide when driven to extremes.
In a mobile, the weight borne by the pivot of the topmost element is the weight of the entire assemblage, whereas the weight borne by the pivot of the bottommost element is only the weight of the bottommost element. Also, the inertial mass rotated by the pivot of the topmost element is the inertial mass of the entire assemblage, whereas the inertial mass rotated by the bottommost pivot is only the inertial mass of the bottommost element. Hence, it is desirable to eliminate extraneous mass, especially in the lower rotatable elements.
Mobiles must have low mass and low friction bearings, especially indoors where wind currents have little force. Mobiles driven by solar cells also must have low mass and low friction bearings, especially indoors under artificial light which has low power.
In an economic design of a mobile, same parts are used throughout, to lower costs by price breaks for mass purchases. Eliminating parts and lessening weight furthers this goal since then all pivots can have the same starting torque ratings and still rotate their loads.
A well-designed, motor driven mobile governs the speed of rotation lest rotatable elements collide with each other or disintegrate by centripetal force. One solution to is to use small motors with reduction gearing, however that adds weight to a mobile.
Solar cell driven mobiles often have solar cells mounted in the base driving a motor in the base. This solution keeps the mass of the solar cells and motors out of the rotating elements but requires complicated mechanical or electrical transmission means to power many rotatable elements.
U.S. Pat. No. 6,906,254 teaches a mobile where each rotatable element is self-contained and capable of producing its own movement. Each rotatable element has a solar cell for power and a motivator for causing movement, for example an electric motor with a propeller. In U.S. Pat. No. 6,906,254, the solar cell is on one end of a beam, and the motivator on the other end. A swivel at the balance point of the beam attaches the beam to other similar rotatable elements. U.S. Pat. No. 6,906,254 teaches that self-contained rotatable element have the advantage of simplicity by eliminating electrical or mechanical transmission between rotatable elements. U.S. Pat. No. 6,906,254 has the disadvantage that it requires a separate swivel and a heavy aerodynamic propulsion device such as a propeller, on each rotatable element. The present invention has fewer parts and is lighter by eliminating the swivel and propeller. In the present invention, the bearing of the motor serves double duty as a swivel between rotatable elements.
Daniel Chadwick “solar kinetic mobile” http://www.youtube.com/watch?v=CADgwsEbOUU discloses a similar mobile, also having self-contained rotatable elements each having a solar cell and a propeller driven by an electric motor. However in Chadwick's mobile, the motor driven propeller is located at the balance point of the beam of the rotatable element. Such an arrangement produces a different motion than U.S. Pat. No. 6,906,254. Chadwick's mobile also has a swivel at the balance point of the beams, attaching the beam of one rotatable element to another. Chadwick's mobile has the same disadvantages as U.S. Pat. No. 6,906,254.
Daniel Chadwick discloses another “solar kinetic mobile” http://www.youtube.com/watch?v=xXTt-h2xz o having the rotating shaft of the electric motor of each rotatable element driving a reduction gear which then drives the next rotatable element. The rotation axis of all rotatable elements are substantially on the same axis. The mobile is suspended from a bearing collar in the middle of said axis. Said axis is essentially horizontal and the planes of rotations of the rotatable elements are substantially vertical. The arms of the solar cells about the axis of rotation are short. Said mobile has the disadvantage that heavy reduction gears are required to limit the speed of the rotatable elements and to overcome the friction of bearings under large bending moments. In the present invention, gearing is not required since the moment of inertia and aerodynamic drag of the solar cells and beam on long arms from the axis of rotation serves to govern the speed of rotation. The present invention also has the advantage that the bearings of the electric motors carry axial gravity loads and have no friction due to bending moments, allowing movement under low power. Chadwick's mobile elements are arranged iteratively (not recursively) on the same axis. The present invention also has the advantage that a mobile element is recursively supported asymmetrically offset from the prior mobile element.
Cerrito and Farinha disclose a “solar chandelier” http://prntscreen.net/thisblog/selected-works/solar-chandelier also having rotatable elements, each having a solar cell and an electric motor. In Cerrito and Farinha's mobile each motor is located at the balance point of a rotatable element and directly rotates the rotatable element below. In Cerrito and Farinha's mobile each motor's bearing serves double duty as a swivel between rotatable elements.
In Cerrito and Farinha's mobile the rotating electric motor shaft is directed downward and connected to the sub assembly below. The electric motor of one rotatable element rotates the sub assembly below. Thus light to the solar cell of one rotatable element does not move said solar cell (and change the light it receives) but only moves the solar cells in rotatable elements below. This has the disadvantage that a rotatable element can not move itself. The present invention has the advantage that a rotatable element can move itself without receiving power or control from another rotatable element.
In Cerrito and Farinha's mobile, rotatable elements are suspended one to another by torsionally rigid hangers, comprising a short beam centered on the shaft of an electric motor having four flexible lines connected from each corner of the short beam to the beam of the connected rotatable element. A hanger does not allow a rotatable element to move itself, even via reactive (equal and opposite torque) effects in the opposite direction of rotation as the electric motor is driving the rotatable element below. A rotatable element is prevented from rotating due to reactive torque of its own motor by the rotatable element's torsionally rigid hanger to the rotatable element above.
If rotatable elements are suspended one to another by a torsionally flexible hanger between one element and the shaft of the motor of another element, when the motor starts the reactive torque effect may wind up the torsionally flexible hanger like a rubber band. The torsionally flexible hanger may wind up until it becomes torsionally stiff, stopping the motor's rotation, then unwinding against the stopped motor, driving continued relative rotation of the connected rotatable elements. This may produce an undesirable, jerky motion of the mobile. Yet a torsionally flexible coupling requires less starting torque from the motor. A torsionally flexible coupling, for intermittent driving of the motor, may also produce a smooth, damped, oscillating motion that continues after cessation of motor driving.
The arrangement in Cerrito and Farinha's mobile, results in a particular pattern of motion of the mobile, described as “fighting” since one rotatable element rigidly moves another. The fighting pattern of motion may not be pleasing to viewers.
Cerrito and Farinha's mobile includes control elements whereby a first rotatable element controls a second rotatable element above, so the first rotatable element can command the second rotatable element to move the first. This has the disadvantage of extra weight for control elements.
In Cerrito and Farinha's mobile each rotatable element is not fully self-contained since to move itself, control signals need to be sent from one rotatable element to the element above. This has the disadvantage that when wires are used to transmit control signals, the wires may become twisted by the rotation of rotatable elements.
Cerrito and Farinha's mobile has the disadvantage that each rotatable element does not have an identical configuration: the bottommost rotatable element needs no motor. The present invention has the advantage that the bottommost rotatable element has a motor like the other rotatable elements.
Cerrito and Farinha's mobile also has the disadvantage that rotatable elements are suspended one to another by axially flexible hangers. Such an arrangement requires careful design and fabrication to keep each sub assembly balanced and aligned so that rotatable elements do not collide with each other.
There are numerous solar helicopter toys for sale, where a symmetrical, self-contained rotor element includes a solar panel and a motor and the rotating element of the motor is attached to the body of the helicopter. The present invention has the advantage that an asymmetrical, self-contained mobile element is recursively supported asymmetrically offset from the prior mobile element.